This invention relates to a light waveform measuring device, which is used to measure, for instance, the life of the fluorescence light which is produced when a laser beam is applied to an object under test.
Examples of conventional light waveform measuring devices having high time resolution are as follows:
A first example of the conventional optical waveform measuring device operates according to the so-called "time correlation single photon counting method". In the method, the photons of fluorescence light emitted from an object under test in response to the application of a light pulse very short in duration time are detected, so that the period of time which elapses from the application of the light pulse until the detection of the photons is measured. In this connection, it should be noted that the device is so adjusted that the number of photons detected per application of the light pulse is not more than one. The time measurement is repeatedly carried out many times (about one million times for high accuracy), and a histogram is formed by using the resultant data, so as to obtain the fluorescence life characteristics of the object under test.
A second example of the conventional optical waveform measuring device employs a streak camera device. The waveform of light emitted from an object under test is measured with a streak tube which performs the sweep of an electron beam in synchronization with the application of a light pulse to the object. By performing the application of the light pulses and the sweep of the electron beam repeatedly the waveform of the light can be measured even when the number of photons emitted from the object is small. In addition, in the case, too, where a number of photons are emitted from the object every application of the light pulse, the light waveform can be measured.
In addition to the above-described devices, a sampling type light waveform measuring device, and devices using a pin photodiode or avalanche photodiode as a photodetector, are known in the art.
In the above-described measurements, it is essential to provide a reference signal for starting a measurement. The light waveform measuring device operating in the single photon counting method cannot operate without a time measurement start signal. The light waveform measuring device with the streak camera cannot operate without a sweep start signal (trigger signal). Therefore, the conventional devices obtain the measurement starting reference signal according to the following two methods: In the first method, the pulse beam from a laser light source is split by a half-mirror or the like into two beams. One of the two beams is applied, as an exciting light beam, to a specimen under test, while the other is applied to a photodetector to obtain the measurement start signal. In the second method, the measurement start signal is obtained by utilizing the signal which is applied to a drive circuit provided for a semiconductor laser constituting a laser light source.
In the first method, however, it is necessary to provide an optical system for splitting the pulse beam into two beams. Accordingly, the light waveform measuring device according to the first method is unavoidably intricate in arrangement. Furthermore, since the pulse beam is split as was described above, utilization of the light beam is not efficient.
In the second method, there arises a drift or jitter between the electrical signal in the drive circuit of the pulse light source and the output pulse beam, so that the relation between timing of the measurement start signal and that of the output pulse beam is not kept constant. Therefore, especially when the counting operation is carried out repeatedly, the timing relation will be shifted every counting operation, with the result that it is difficult to improve the time resolution.
On the other hand, in order to measure the life of fluorescence, it is necessary to use a light beam as an exciting light beam which is shorter in wavelength than the fluorescence light, and laser light source types which can be utilized are therefore limited. In the case of a semiconductor laser which is small in size and can be controlled with ease, it is rather difficult to obtain a laser beam having a short wavelength.